1. Field of the Invention
This invention relates to a process for reducing the concentration of selenium ions in aqueous systems and more particularly to removal of selenate ions from leaching solutions or mine water prior to discharge of the aqueous stream into the ground water system.
2. Description of the Prior Art
Pollution and pollution control have become increasingly more important in recent years, not only because people are more sensitive to their environment, but also because the amount of pollution has increased markedly in some instances. Pollution represents a potential health hazard and a deterioration in the quality and beauty of our surroundings. It is one of the undesired consequences of our rising production, our modern civilization and out increased utilization of natural resources.
Water is mankind's most valuable resource. Any use of water nearly always results in a decrease in the quality of the water through the addition of pollutants. These pollutants can comprise dissolved inorganic and organic chemicals, inorganic and organic particulate material and absorbed gases. In general, several methods are available for removing pollutants. These methods include such processes as gravity sedimentation, flotation, filtration, biological oxidation, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation and chemical precipitation. Several soluble inorganic pollutants present a problem that generally is not solved by gravity sedimentation, flotation, filtration or biological oxidation.
Stringent standards for the maximum level of pollutants in water to be used for drinking or released into ground water system are being promulgated by federal and state agencies. For example, the current allowable maximum concentration level for selenium in drinking water set by federal standards is 0.01 milligrams per liter. The state of New Mexico has proposed a selenium standard of 0.05 milligrams per liter for discharge into the ground water system of the state.
An increase in pollutant concentration is, in itself, not significant provided that processing methods are available to reduce the pollutant concentration to an acceptable level. One method employed to remove or substantially reduce the concentration of soluble inorganic pollutants such as heavy metals in water is chemical precipitation of the metals as oxides or hydroxides. This precipitation generally is effected by the addition of lime, alum or an iron salt to the water at an appropriate pH.
Other treatment methods, such as ion exchange, reverse osmosis, electrodialysis or distillation also can be effective in removing various pollutants. However, these methods are considerably more expensive and generally narrower in applicable scope than is desirable for the treatment of great volumes of water as is necessitated in many industrial operations.
It is known that selenium ions can be removed from aqueous systems employing chemical precipitation if the selenium is present in the selenite or Se(IV) oxidation state. The percentage of selenite ions removed from an aqueous solution has been found to be better when an iron salt is employed to effect the chemical precipitation rather than lime or alum. The iron salt can be present either as a ferric or ferrous sulfate, chloride or hydroxide.
In particular, experimental studies have shown that chemical precipitation employing ferric sulfate can achieve a significant removal of selenium in the selenite oxidation state from an aqueous stream. More particularly, when a river water containing 0.03 milligrams per liter of selenium in the Se(IV) oxidation state and having a pH of 5.5 is treated with 30 milligrams of ferric sulfate per liter, about 85 percent of the selenium is removed from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May, 1977, pages 29-31).
U.S. Pat. No. 3,933,635 discloses a process for removing selenium ions present in the selenite oxidation state from acidic process waters. The acidic process water, having a 1.0 to 4.0 pH, is reacted with a metallic reducing agent at a temperature in the range of from about 25.degree. C. to about 85.degree. C. for a sufficient time to reduce the soluble selenium in the selenite oxidation state to insoluble elemental selenium. Preferably, the temperature is maintained in the range of from about 50.degree. C. to about 70.degree. C. The reducing agent can comprise aluminum, iron or zinc in an appropriate form, such as, for example, a powder, scrap fragments, granules, wools and the like. The preferred reducing agent for selenium in the selenite oxidation state is zinc powder.
By way of contrast, laboratory tests and pilot plant studies have shown that chemical precipitation employing alum, lime, ferrous sulfate or ferric sulfate substantially is ineffective for removing selenium in the selenate or Se(VI) oxidation state from water. Studies on water having a 0.03 to 10 milligram per liter selenium concentration in the Se(VI) oxidation state have shown that the conventional chemical precipitation methods remove less than 10 percent of the selenium from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May, 1977, pages 29-31).
It has been shown that the selenium ions in the selenate oxidation state can be removed by ion exchange or reverse osmosis. As previously indicated, these methods are prohibitively expensive when large volumes of an aqueous solution must be treated. Further, both methods produce a contaminated regeneration effluent that requires further treatment for selenium fixation or removal before disposal. Thus, the selenium removal problem still exists but in a more highly concentrated solution.
In view of the various problems associated with the removal of selenium ions in the selenate or Se(VI) oxidation state it would be desirable to develop a process by which the selenate ions can be removed by chemical precipitation.